Single-walled carbon nanotubes (SWCNTs) have a unique one dimensional structure with extraordinary thermal, mechanical, electro-optical and electronic properties making them promising candidates for various applications such as electronic devices, chemical sensors or hydrogen storage devices (see, for example, Jorio, A., Dresselhaus, G., Dresselhaus, M. S., Eds. Carbon Nanotubes, Advanced Topics in the Synthesis, Structure, Properties and Applications; Springer: Berlin, 2008; foreword, pages V to IX).
Electronic properties of SWNTs depend on their geometry, i.e. diameter and chirality. Each tube structure can be identified with a pair of integers (n,m), which illustrates how the graphene is rolled up to form the nanotube. These chiral indices (n,m) specify the perimeter of the carbon nanotube (chiral vector) on the graphene net. Thereby the integers (n,m) also determine diameter and helicity of a carbon nanotube. If m=0, the nanotube is called “zigzag”. If n=m, the nanotube is called “armchair”. Otherwise, the nanotube is called “chiral”, since in such cases the chains of atoms spiral around the tube axis instead of closing around the circumference.
The optimal performance of SWCNTs in many potential applications relies on the (n,m) monodispersity of tube samples, because SWCNTs of different (n,m) structure have distinct properties. Most SWCNT synthesis methods result in tube samples with a wide (n,m) distribution. Studies have demonstrated that SWCNTs with narrow (n,m) distribution (less than 20 species) can be produced, and moreover, the (n,m) selectivity can be manipulated by optimizing growth conditions, such as temperature, catalyst support, carbon feedstock, gas pressure, and crystal plane. Further, as the SWNT diameter decreases, the number of possible selections (n,m) on the graphite sheet for forming SWNT decreases, so that the diversity of possible chiral conformations decreases. Accordingly, small diameter SWNTs with a narrow diameter distribution are highly desirable, since the respective SWNTs have more uniform electronic properties. Beyond all those growth conditions, catalysts play the most crucial role in determining the (n,m) distribution of SWCNTs produced. The development of new catalysts which can lead to large scale and economical production of SWCNTs with desired (n,m) structures is the premier target in SWCNT synthesis research.
Several catalysts have demonstrated good selectivity toward narrow (n,m) distribution SWCNTs, which include Co/Mo catalysts (Bachilo, S M, et al., J. Am. Chem. Soc. (2003) 125, 11186), Fe/Co catalysts (Maruyama, S, et al., Chemical Physics Letters (2002) 360, 229), Fe/Ru catalysts (Li, X., et al., J. Am. Chem. Soc. (2007) 129, 15770), and Co-MCM-41 catalysts (Lim, S, et al., L., J. Phys. Chem. B (2003) 107, 11048; Ciuparu, D, et al., Journal of Physical Chemistry B (2004) 108, 10196; Ciuparu, D, et al., Journal of Physical Chemistry B (2004) 108, 503). The first three catalysts are all bimetallic catalysts. Synergism effects between two metallic species help stabilizing metallic clusters, which enable narrow distributions of (n,m). An efficient catalyst for economical nanotube production also requires the simplicity of removing substrate and metallic clusters in the follow-up nanotube purification process. From this point of view, a mono-metallic catalyst is preferred to its bimetallic rivals, because Mo or Ru compounds are difficult to remove from nanotube samples. Ciuparu et al. (2004, page 10196, supra) successfully incorporated mono-metallic Co into a mesoporous molecular sieve (MCM-41), and use it as a catalyst for SWCNT growth (Ciuparu et al., 2004, page 10196, supra; Ciuparu et al., 2004, page 503, supra; Chen, Y, et at, Carbon (2006) 44, 67). Other metals such as Ni and Fe can be also incorporated into MCM-41 for SWCNT growth. The narrowest (n,m) distribution from a bulk SWCNT sample has been reported on tubes produced from a Co-MCM-41 catalyst. This catalyst also enables a mild, four-step purification method to obtain low-defect tubes. However, the drawbacks of Co-MCM-41 catalyst are their high cost (various expensive surfactants), long synthesis time (7 days in autoclave), and relative low carbon loading (1.25 wt. % carbon/1 wt. % cobalt), which significantly further increases the cost of SWCNTs. It is therefore desired to obtain a novel mono-metallic catalyst, which can retain a good selectivity toward narrowly (n,m) distributed SWCNTs, and that at the same time can be synthesized at a lower cost and in shorter time compared to Co-MCM-41. Ideally such a catalyst can produce SWCNTs with higher productivity.
It is therefore an object of the present invention to provide a method of producing single-walled carbon nanotubes that avoids the above described drawbacks or shortcomings of the current techniques.